Authenticate requests solely from the Authorization: Bearer header on the
server; stop accepting the legacy `token` header and `usertoken` cookie.
The frontend no longer sets auth cookies (only clears legacy ones), omits
credentials on all requests, and drops the redundant `token` header. Project
event SSE now streams over fetch so it can carry the Authorization header,
which EventSource cannot.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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